Process for carbon structure control in iron



Jan. 1, 1935.

7 w. J. KELLY PROCESS FOR CARBON STRUCTURE CONTROL INIRON Filed Dec. 19,1950 INVENTOR. W/LL/AM J. KELLY ATTORNEY.

Patented Jan. 1,1935

UNITED STATES PATENT OFFICE PROCESS FOR CARBON STRUCTURE CONTROL IN IRON3 Claims.

The presence of carbon in iron and alloys thereof results in widelyvarying metallic structures in the castings produced. Thecharacteristics of strength, hardness, density, heat resistance, andresistance to wear, shock, stresses and the like are all directlydependent in part on the condition in which the alloyed carbon exists inthe metal. V

This invention relates generally to the structure of carbon in itsvarious forms and alloys with iron. It provides a practicaland'eflicient method for. controlling the combined and graphitic carboncontents and permitting any desired percentages of these two elements inmanufacturing iron castings for a wide variety of purposes.

In particular it relates to a process of heat treatment for ironcastings and especially with the purpose of controlling the carbon formand its manner of association in the resulting metallic structure. Aspecific application of my invention is the manufacture of plungercastings for making glass tumblers and the like.

The primary object ofthe invention resides in 5 ,the production ofcastings for a wide variety of processes and also relative purposes bycontrolling the heating and cooling in relation to the time involved ineither of these to the carbon states and content in the alloy. A furtherobject is to provide an accurate control of the heating and the mannerand rate of cooling ofthe castings for the purpose of controlling thecarbon structure.

As a preliminary step to the treatment hereinafter described a castingis made in the form in which the metal is to be finally used and ofproportions as are presently to be explained.

Various purposes necessitate diflerent quantities of the'elementsentering into the alloy as well as quantities of added elementsincluding chromium, vanadium and nickel or combinations thereof, alsotitanium, tungsten and molybdenum.

In general the constituents of the alloy are those essential to theproduction of white iron and the constituents will limits: carbon from3.00 to- 4.00 per cent; silicon .5to 1.00 per cent; manganese .70 to1.50 per cent; phosphorus 0 to .20 per cent and sulphur 0 to .10 percent. Ingeneral the carbon in. the castings is in the form of cementiteor in solid solution. ,The combined condition of the carbon is reaattained.

As distinguished mm the heat treatment of ordinary gray cast iron inwhich only .9 per cent vary in the followingof the carbon content of theiron is taken into consideration for treatment under this inventioncarbon is formed in one condition alone, and then by a heat treatment athigh temperatures the uniform carbon compound is decom- I posed in freeor temper carbon and the subsequent control of the temperaturedetermines the condition in which the carbon reenters into combinationwith the iron. Where carbon exists in the castings in two forms abovethe critical 10 range a portion of the free carbon enters solid solutionand slow cooling through the critical range allows the carbon to producethe pearlitic structure. Below the critical range the finished productconsists of ferrite and cementite plus 16 mechanically mixed carbon inthe form of graphite. Where all of the carbon is formed in solidsolution in the castings, the heat treatment precipitates a quantity oftemper carbon and slow cooling through the critical range per- 20 mitsthe remainder of the carbon to combine with the iron and form cementite.In either case the results are identical and the treatment is applicableto all classes of castings irrespective of the carbon condition. 1

The metal for the casting to be treated may be melted either in acupola, air or open hearth furnaces or electric furnace and may bepoured into sand molds provided the iron mixture is suitable and thesection of the casting light, or so of such thickness that the rate ofcooling through the freezing or setting point of the metal will besufliciently rapid to produce castings in which substantially all of thecarbon is in solid solution. For castings of heavier sec- 35 tion it isnecessary to pour the metal into iron chill or permanent molds so thatthe cooling rate will be extremely rapid through the freezing period andthe resulting castings white.

Since only a portion of this invention admits 40 of illustration aspecial form of such a casting is illustrated in the annexed drawing inwhich The figure is a side elevation of a completed casting speciallyadapted to use by the process hereinafter described.

For convenience of illustration the casting 1 represents a plungercasting for making glass tumblers whose physical requirements of closegrain density, high machinable hardness, high resistance to-heat andoxidation and freedom 50 from scaling-has thus been prepared. When thecasting 1 is removed from the mold its outer surface is black and inthis condition the casting is removed to a furnace for treatment. In thecase of the plunger castings hereinafter specially seer cert described,the castings are given an immersion in milk of lime to protect the outersurfaces.

Any type of furnace is adequate for the heat treatment to which thecasting is subject but the appropriateness of the furnace to the shapeof the casting should be observed in its construction. Adequate controlof the temperatures to be attained is a requisite of the process as wellas the provision of means for observing the temperatures. For thislatter purpose an indicating pyrometer is preferably employed.

The control of the combined carbon in the iron casting is the result ofthe two elements of temperature and time and depends on a carboncondition which may be readily controlled. Temperatures up to 1300 F. donot materially affect the iron carbide of white chilled cast iron but Ihave found that with'increasingtemperatures' the rate of the carbonprecipitation becomes very rapid until at- 1750 F. the rate is veryrapid and breaks down the combined carbon content to a percentage whichwill enable the casting to be machined. The furnace in which the castingis treated should be able for this reason to attain temperatures of 2000F.

and above. 7

The temperature is gradually raised requiring from 40 to 45 minutes toapproximately 1750 degrees. It is maintained at this temperature forabout fifteen minuteseand then allowed to cool in the furnace toapproximately 1250 F. in the same gradual manner substantially as thetemperature was raised. Various modifications in the treatment can bemade depending upon the section and weight of the castings which arebeing treated; in general, thin castings require a shorter treatmentthan heavier ones. In this manner castings may be produced whosehardness may vary from 800 Brinell to 100 Brinell. I

The treatment is, of course, not limited to slow cooling and I havefound that iron castings prepared as in this process may be quenchedfrom temperatures of 1700 F. to 1100 1?. With such treatmenttemperatures above the critical temperature, or approximately-1300 F.produce hardness while below the hardness of the annealed samples isunaltered. Iron hardened in this manner may be restored to its normalstate before tempering by heating to the critical temperature andpermitting it to cool slowly. While similar treatment administered toordinary gray iron producm hardening, iron produced by this processresponds more readily and in a greater degree.

A semi-malleable product may be produced by maintaining the castings ata high temperature for three or four hours and then allowing them tocool to 1100 or 1200* F., at which temperature they are maintained forapproximately two hours. For such treatment the castings are placed inannealing boxes and surrounded with. charcoal or lime. The iron thusproduced is more ductile, tougher and its tensile and transversestrengths are improved.

For special purposes, ,as suggested, various alloys are formed toeifect'the desired result in -:combination with the carbon; Under thisman- K such as are'used in casting glassware but satisner of treatment Ihave found castings of the following analyses unsuited for plungercastings mium .16 to .30, nickel .08 to .25, graphitic carbon 2.83 to1.70 and combined carbon .87 to 2.11. Nevertheless, an alloy comprising.90 silicon; sulphur .07, phosphorus .07 to .10, manganese .73, chromium.16, nickel .10, and 3.60 carbon, is admirably suited for treatment bythis process to produce plunger castings. The low sulphur and phosphoruscontent insures high' heat resisting qualities. Chromium in this amountproduces a double carbide of iron and chromium which does not break downin the least under exposure to the relatively high temperatures to whichthe casting is subjected during treatment, while the nickel assists inproducing a fine grain throughout the casting. Small percentages ofnickel are more suitable for this purpose, I have found, than largerquantities. The percentages of silicon and carbon are proportioned toproduce in the iron a strong hard chill with a high'combined carboncontent insuring a minimum of shrinkage upon solidification and therebyavoiding disintegrating tendencies under heat characteristic of ironshaving high coeificients of expansion.

However, I specific analyses outlined for specific purposes since theprocess is applicable to all metals within the limits previouslysuggested including substantially all types of iron castings. In allcases, it is the percentages and distribution of the combined andgraphitic carbon which are of importance. The carbon content madeuniform and homogeneous throughout all areas of the casting by thisprocess insures homogeneity of physical characteristics of strength,hardness, grain density, heat resistance, resistance to wear, shock,stresses and the like.

Cast iron made and treated by this process, I have found, has improvedphysical ualities and iron castings can be manufac d having a muchhigher'machinable hardness castings made by other methods.- It sho d beobvious that the graphitization process can produce castings in additionto the purposesf noted which are adaptable as gears, rolls, chemicalcastings and machine and automobile castings.

Having thusdescribed my invention what I claim as new and desire tosecure by Letters Patent is:

1. The process of carbon control in iron castings which consists inpouring metal having a do not desire to be limited to thecarbon contentsubstantially from 3.00 to 4.00

per cent, in a chill mold, reheating the casting in a maximum period ofapproximately fortycasting through a maximum period of approximatelyforty-five minutes to approximately 1250 1". and cooling the castingfather to room temperature.

2. The process of carbon control in iron castings adaptable as plungersand the like which consists in pouring metal comprising substantially.90 per cent silicon, .07 per cent sulphur, .07 to .10 per centphosphorus, .7 per cent manganese, .16 percent chromium, .10 percentJnickel and 3.60 percent carbon into' a chill mold,

covering the casting with aprotective coatin slowly reheating thecasting 1750' F., maintaining the casting at this temperature fora shorttime and permitting-the casting to cool slowly substantially as it was8. The process of carbon to approximately 70 control in-iron castis ingsadaptable as piungers and the like which tially forty-five minutes toapproximately 1750* consists in pouring metal comprising substan- F.,maintaining the temperature for approxitially .90 per cent silicon, .07per cent sulphur, mately fifteen minutes permitting the casting to .07to 1.00 per cent chromium, .10 per cent nickel cool to approximately1250 F. throughout a like 5 and 3.60 per cent carbon into a chill moldcoverperiod of approximately forty-five minutes, and 5 ing the castingwith a protective coating, slowly further cooling the casting to roomtemperature. reheating the casting over a period of substan- WIILIAM J.KELLY.

